After running X -ac from a terminal window (X was closed before) I was able to use SET DISPLAY/CREATE/... and then RUN without problems. I did not connect with ssh -X or ssh -Y (in fact, being on a "secure" LAN, I used telnet) I am aware that it is not the best policy to simply run an X server without any authentication on any network; but I think it would at least be useful for some diagnostics.